The invention concerns a hydraulic system with a pressure source controllable via a load sensing signal, a pressure sink, at least two working sections, each having a hydraulic consumer and a control valve with a load sensing connection, and at least one back-pressure valve arranged in a tank pipe between the control valve and the pressure sink.
Such a system is known from DE 42 35 762 C2.
In this case, the pressure source can be a pump with controllable discharge capacity. However, it is also possible to provide a pump followed by a pressure control valve.
In many cases a proportional valve is used as control valve. In the neutral position of this valve the load sensing signal connection is connected with the tank pipe. The load sensing signal can also be called load pressure signal. The load sensing signal connections of all working sections are connected with each other via shuttle valves in a way that the load sensing signal with the highest pressure reaches the controllable pressure source. Thus the pressure source can produce the required pressure corresponding to the load sensing signal, also called LS-signal. The fact that the load sensing signal connection is connected with the tank pipe in the neutral position of the control valve should secure that without a consumption from a consumer the pump does not produce a higher pressure. When the control valve is in its neutral position, the hydraulic consumer connected with the control valve, i.e. a motor or a piston-cylinder unit, is not influenced and accordingly it needs no hydraulic output.
However, a certain problem occurs because of the back-pressure valve. When a hydraulic consumer, e.g. a piston-cylinder arrangement with two working chambers, is loaded by an external force leading to a displacement of the piston of this piston-cylinder unit, one working chamber must be expanded, the other must be reduced. This is for instance the case with front-end loaders, the loaded shovel of which must be lowered. In the expanding working chamber there is a relatively low pressure, e.g. 0 bar. To avoid cavitation damages, additional hydraulic fluid should be supplied at a correspondingly low pressure. However, this additional supply should not lead to an increase of the force acting on the piston. The additional supply takes place through a refill valve arranged between the two working chambers of the consumer. To overcome the closing force of this refill valve, it is necessary that a certain pressure builds up on the corresponding side. The building-up of this pressure is secured through the back-pressure valve. The back-pressure, i.e. the pressure before the back-pressure valve, is in such cases normally fairly close to a load sensing pressure, thus corresponding to the load sensing signal. Due to the pressure drop over the refill valve, certain differences will, however, occur. This causes that the load sensing pressure on this consumer is normally lower than the back-pressure. As the higher pressure is always regarded as load sensing pressure, the back-pressure will be reported back to the pump control. This leads to an increase of the pump pressure. This again affects the back-pressure, which becomes smaller. When the back-pressure becomes smaller, the load sensing signal reassumes the control of the pump. Hereby the pump pressure becomes lower and the back-pressure becomes higher, leading to the initially described situation. There is a risk that the system starts oscillating and unstable conditions occur.